English: Material wastage on construction sites can contribute to cost overruns. Research to provide evidence of the extent of material wastages’ contribution to cost overruns on construction sites is based mostly on surveys. Thus, the actual contribution is not yet ascertained. The purpose of this article is to report the results of an objectively investigated study on the contributions of material waste to project-cost overrun. The methodological approach adopted for the study is the quantitative technique that is rooted in the positivist paradigm. The investigation included ongoing building construction projects within Abuja, Nigeria, from which a sample of 31 public and private projects was purposefully selected (project value of ₦1.6 billion Naira and above). The data for this research were sourced from the field investigation (measurement of the volume of material waste) and data from the archival records (drawings, bills of quantities, project-progress reports, and specifications) on material waste and cost overruns. The collected data were analysed using the Pearson moment correlation and the descriptive method. The research results revealed a statistically significant relationship between material waste and cost overrun. This implies that any increase in the volume of material waste would lead to a corresponding increase in the amount of cost overrun. The results showed that the significant percentage contribution of material waste to project-cost overrun ranges from 1.96% to 8.01%, with an average contribution of 4.0% to project-cost overruns. It is recommended that construction professionals be well informed of the consequences of material waste contributions to project-cost overrun at an early stage, in order to enable the professionals to evaluate the extent to which these consequences could be minimised.